The project applies the techniques of experimental nuclear and particle physics to the development and enhancement of imaging devices to observe and follow the metabolism of biological materials tagged with the isotope 125I in mice. Position sensitive photomultiplier tubes are coupled to finely segmented (pixellated) scintillators to provide images with spatial resolution of 1-2 mm. Specially designed collimators are used to provide unmagnified or magnified images with a range of spatial resolution and efficiency based on the chosen design parameters. These detectors are used to provide in vivo images of the internal distribution and elimination of the chosen biological materials over periods of minutes to days without need to sacrifice the animal being studied. The ready availability of many biological materials tagged with the isotope used here (125I) provides excellent opportunity for innovative studies by collaborating biologists. An important added aim of this research is the opportunity provided to undergraduate physics majors to become involved in research that applies the tools of physics and mathematics to medical/biological research. William and Mary is a selective public institution which has enjoyed an excellent record in training premedical students. This project provides the first organized effort to involve physics undergraduates in research in the health fields. Although this proposal is primarily directed toward detector development, the physics students will continue to benefit from collaboration with our colleagues, both faculty and students, in Biology. Examples of some of the biological studies facilitated by these detectors are described in the proposal. Between the years 2000 and 2004,12 undergraduate physics majors have carried out their research project in the work described here. Eight of those undergraduates have presented papers at national scientific meetings. This proposal requests renewal of support to continue and to enhance these goals. [unreadable] [unreadable]